Current drivers are used to drive currents through a load, such as, but not limited to, a laser diode type load. Many current driver circuits, such as those shown in FIGS. 6 and 11, include metal oxide semiconductor (MOS) devices, also referred to as MOS transistors. Accordingly, it is useful to first review of the characteristics of MOS devices and laser diodes, in order to understand their impact on circuit topology.
In FIG. 1 is shown the symbol and nomenclature for a PMOS device (also known as a PMOS transistor) and an NMOS device (also known as an NMOS transistor). In FIG. 2 is shown simplified equations governing the relationships between the applied voltages and the supplied currents of MOS devices. In FIG. 3 is shown some arbitrarily sized NMOS and PMOS devices. This is a very simplified model, and in the real case there are many factors that modify the results. However, the modifications to the results are moderate, and the simplifications shown will not invalidate the discussion and comparisons below.
Simply expressed, a MOS device functions like a resistor if VDS is less than VON, (where VON=VGS−Vth). But if VDS is greater than VON, (where VON=VGS−Vth), the MOS device functions like a current source. The value of the resistor is inverse to VON−VDS. The NMOS device can be smaller than the PMOS device for the same characteristics by the ratio of the mobility terms U, where the mobility is dependent on the semiconductor doping. The ratio of the gate width to the gate length can be varied by the designer to obtain devices of different size. Usually the gate length is the minimum allowable length, so it is the width that is increased to obtain a device capable of more current. In other words, relative sizes of MOS devices are typically descriptive of the gate width.
FIG. 4 is an exemplary graph showing the relationship between laser current and laser optical power output for a laser diode. FIG. 5 is an exemplary graph showing the relationship between current and laser voltage for a laser diode. It can be appreciated from FIG. 5 that for very low currents, the laser voltage increases rapidly with increasing laser current. Also, by the time the current is large enough to cause the laser diode to laser, the laser diode acts like a battery and a series resistor. Also shown in FIG. 5 is an exemplary VDD voltage for various circuits discussed below. The headroom voltage is the difference between VDD and the laser voltage.
FIG. 6 illustrates an exemplary current driver 602, according to the prior art, which shall be referred to as a current mirror current driver because of its use of a simple current mirror 604. The current mirror 604 includes transistors Q1 and Q2, which are connected in a common source configuration and a common gate configuration. The gate and the drain of transistor Q1 are connected together, to form the input of the current mirror 604, and the drain of transistor Q2 forms the output of the current mirror 604.
A control current source Icontrol is connected between the input of the current mirror 604 and a voltage rail, which is shown as being ground (GND). A two level control signal is used to control the current source Icontrol, i.e., to turn the current source Icontrol off and on (or from a first current level to a second current level). When the current source Icontrol is turned on, the current produced by the current source is mirrored by the current mirror 604, such that a proportional current is produced at the output of the current mirror 604, i.e., at the drain of transistor Q2.
In this configuration, the output of the current mirror 604 is the output current (Iout) of the current driver 602. This output current Iout is shown as being used to drive a laser diode (LD), alone, or together with one or more other driver(s), which is/are represented by block 606. In other words, the output current Iout of the current mirror current driver 602 is added to current(s) produced by the other driver(s) 606, to thereby produce the total output current at the IOUT pin, which is used to drive the laser diode LD.
The current mirror current driver 602 is very simple, but suffers from a speed versus power consumption (i.e., efficiency) problem. More specifically, if the size of transistor Q1 is made equal to the size of transistor Q2, then the current mirror current driver 602 is fast. However, when the size of transistors Q1 and Q2 are equal, the output current flows not only to the load (LD in this example) from transistor Q2, but also a copy of the output current flows through transistor Q1, resulting in an efficiency of less than 50%. To make the efficiency higher, the size of transistor Q1 can be made much smaller than transistor Q2. However, when transistor Q1 is much smaller than transistor Q2, the applied control current Icontrol must charge the sizeable capacitance seen at the gate of transistor Q2, causing the circuit to be slow. Accordingly, the current mirror current driver 602 can be fast or be efficient, but not both.
FIGS. 7-10 are used to shown the conduction characteristics for various MOS devices. In these FIGS. the variable D is a multiplier that is useful for making the plots. Also, in the legend of these FIGS, V is representative of VON.
In FIG. 7, there is shown the conduction of a small MOS device (beta=0.05) with a large VON. There is also shown a load line (ILOAD) from a high power laser diode. It can be seen that small currents can be obtained with VON of less than 1V, and that the device is then in the current source mode of operation. But if VON is further increased, the device moves to the resistive mode of operation. In this case, the laser current becomes a function of the supply voltage, laser voltage, laser resistance, and MOS device resistance. If the small MOS device of FIG. 7 were used in the current mirror current driver 602 of FIG. 6, the accuracy and predictability of the Iout current would be compromised, by the variations of the power supply, and the change in operation mode of the MOS device from current source mode to resistor mode. In addition, if other currents (e.g., from other drivers 606) were added to Iout, the voltage at the IOUT pin would increase and also effect the current through transistor Q2.
In FIG. 8, there is shown the conduction of a large MOS device with a small VON. The MOS device is 6 times larger (beta=0.3) than the MOS device of FIG. 7 (where beta was 0.05), but is able to achieve the same output current with only 1V of VON. The difference here is that the MOS device is nearly always in the current source mode of operation, and not as sensitive to the supply voltage changes. If the large MOS device of FIG. 8 were used in the current mirror current driver 602 of FIG. 6, as long as the voltage at the IOUT pin remains small enough, other currents (e.g., from other drivers 606) can be added to Iout without significantly changing the current through transistor Q2. This would be the typical sizing of MOS devices for the current mirror current driver 602 of FIG. 6.
In FIG. 9, there is shown the MOS device of FIG. 7, but with the load line of a 2.9 ohm resistor to ground. Here it can be seen that the maximum current is limited by the resistor. It can also be seen that the maximum current is limited by the MOS device to about 2.5 amps.
In FIG. 10, there is shown the MOS device of FIG. 8, but with the load line of a 2.9 ohm resistor to ground. Here it can be seen that this MOS device can supply more current than the MOS device of FIG. 7, but to do so it must have it's VON raised to about 2V. Again the current is limited by the resistor. This MOS device could have a very large output current if the output current were shorted and VON was raised to about 5V.
Referring to FIGS. 9 and 10, it is not surprising that a larger MOS device can supply more current than a smaller MOS device to the same resistor. But, given that the large MOS device is 6 times larger than the small MOS device, the increase in current is not that significant with the normal resistive load. This suggests that from the standpoint of limiting Iout under abnormal conditions, the smaller MOS device with higher VON is better, although there is more power per unit area dissipated in the smaller device.
Returning to FIG. 6, it can be seen that as the headroom voltage (VDS in FIG. 1) decreases due to increasing output voltage, the VON of transistor Q2 must decrease for transistor Q2 to remain in the current source mode of operation. To do so, the size of transistor Q2 must be made larger as the maximum applied output voltage increases. This has the problem of increasing the size of the capacitance of transistor Q2, which will tend to slow down the circuit. Thus, the decreasing headroom has a very high price to pay in terms of MOS device size and circuit speed, if transistor Q2 is to remain in the current source mode of operation.
The current mirror type of laser driver of FIG. 6 was replaced with a faster circuit as CD writers gave way to faster DVD writers. But, it can still be used to provide a read current where there is not a high speed requirement. Accordingly, in order to obtain speed and simple control, a cascode configuration current driver 1102 is often used, as shown in FIG. 11.
Referring to FIG. 11, the cascode switched current driver 1102 includes a pair of transistors Q1 and Q2 that act like a current mirror, with the control current source Icontrol still being connected between the input of the current mirror and GND. Also included in the driver 1102 are transistors Q3 and Q4, which in this arrangement functions as resistors. More specifically, because the gate of transistor Q3 is connected to GND, VGS and VON of transistor Q3 is very large, and VDS of transistor Q3 is very small, causing transistor Q3 to operate in resistor mode. The gate of transistor Q4 is connected to the output of an inverter U4. When the output of the inverter U4 is low (i.e., the input to the inverter is high), the transistor Q4 is on and acts as a small resistor, and depending on the bias determined by the control current and transistor Q1, results in a large current flow through transistor Q2. When the output of the inverter U4 is high (i.e., the input to the inverter is low), the transistor Q4 is turned off and as a result no current can flow through transistor Q2.
In the cascode switched current driver 1102 of FIG. 11, the speed can be increased and the circuit can have high efficiency, but the sizes of transistors Q2 and Q4 become problematic. This is because transistor Q4 uses up some of the headroom voltage. It is as if the VDD has decreased due to the presence of transistor Q4 and the voltage drop on transistor Q4. This reduces the headroom available to transistor Q2, requiring that it's VON must be made smaller, and that transistor Q2 must then be made larger to compensate. So although the cascode switched current driver 1102 can be fast and efficient, it suffers from larger sized MOS devices that must operate with reduced headroom and VON. This increased size increases the output capacitance, causing the circuit to slow-down. Additionally, because of the low headroom, the voltage available to drive the parasitic inductance between transistor Q2 and the laser diode (LD) is decreased, thereby further decreases the speed of operation. Further, since VON is small to keep transistor Q2 in a current mode, the circuit becomes sensitive to noise on the gate voltage of transistor Q2. This is also a problem for the simple current mirror of FIG. 6 when the headroom is small.
Accordingly, there is still a desire to provide a current driver that can operate at low headroom, using smaller devices with low capacitance, and preferably with reduced susceptibility to noise.